What is the contribution of stubble burning to Delhi’s pollution?


In an affidavit filed in the Supreme Court, the Union ministry of environment, forest and climate change has said that stubble burning is not a major contributor to high levels of pollution in Delhi. “As per a scientific study carried out and to get indicative figures to show average sectoral contributions in PM 2.5 and PM 10 concentrations in Delhi” the respective contribution of agricultural burning to PM 2.5 and PM10 was just 4% and 7% each in winter and summer, the affidavit said. An HT analysis of pollution data and scientific research shows that these numbers don’t tell the real story. Here are four charts which explain why.

Have farm fires contributed just 4% to Delhi’s current pollution levels this winter?

Between October 20 and November 14 this year, Delhi’s Air Quality Index (AQI) has been in the severe zone (401-500) on seven days. On each of these days, the contribution of farm fires to Delhi’s PM 2.5 contribution was between 26%-48%. These numbers may fly in the face of the claim made by the Union ministry of environment in the Supreme Court that seemed to suggest that stubble burning by farmers in Punjab and Haryana was not really the problem for Delhi’s bad air. Because Safar data on contribution of farm fires – it is the only publicly available data source – is available only from October 20, this analysis cannot be carried out for a longer period.

See Chart 1: Delhi AQI, PM 2.5 and contribution of farm fires to PM 2.5

So where did the government get its 4-7% figure from?

That is most likely an average number for the entire year. If the same logic were to be applied, pollution is not as big a problem in Delhi. The same affidavit shows that between 2016 and 2020, the number of “severe” AQI days in Delhi was never more than 25. Even if one were to add the number of “very poor” AQI days (readings between 301 and 400) to the number it comes between 64 (2020) and 92 (2016) during this period. Clearly, days with higher AQIs are a bigger problem than days when the AQI is not so bad. And, as has been shown above, contribution of farm fires is significant when AQI levels are higher.

See Chart 2: Days by AQI categories in Delhi

Spike in farm fires coincides with rise in AQI levels

Unlike AQI and PM 2.5 numbers, data on the contribution of farm fires to PM 2.5 levels in Delhi is not available for the period before October 20. What we do have is daily data at a high resolution from 2012 onward on the number of fires seen in Punjab and Haryana from satellites. Reading this data with PM 2.5 levels in Delhi from 2018 onwards (air quality stations were much fewer before 2018) shows that there is a broad resonance in the period when farm fires and pollution levels peak in the city.

See Chart 3: Daily AQI and number of farm fires

Low contribution of farm fires to pollution is an aberration, not the norm

To be sure, there does exist research to show that even on days when farm fires are rampant, their contribution to Delhi’s PM 2.5 levels can be low. However, this is more an aberration due to favourable climatic conditions than proof of farm fires being benign for Delhi’s pollution levels. A 2018 article, Objective evaluation of stubble emission of North India and quantifying its impact on air quality of Delhi, published in the international journal Science of the Total Environment gives a useful insight into this question. “Although the stubble burning was a regular process during the said period (October 10-December 10, 2018), with varied fire count, the impact of biomass burning on Delhi’s PM 2.5 is found to vary on day -to day basis with percentage contribution of stubble burning ranging from 1% to 58% as it is highly dependent on transportation pathway of air mass, controlled by source to target region meteorological parameters led by wind direction and wind speed,” the paper says.

“The impact of stubble burning on Delhi’s air quality depends not only on amount of biomass burned in Punjab and Haryana but a combined effect of high wind speed at intrusion height, wind direction and air residence time in the Delhi airshed,” the paper says.

Given the fact that no policy can control meteorological factors such as wind speed and direction, it makes eminent sense to make efforts to control farm fires rather than hoping for a divine intervention on the meteorological front. None of this, of course, is to say that other factors are not responsible for pollution in Delhi, or other steps need not be taken.

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